Rational syntheses of a variety of porphyrinic compounds bearing diverse patterns of meso substituents have been developed recently. The porphyrinic compounds include porphyrins,1-3 chlorins,4 corroles,5 and bilanes.6 The syntheses begin with dipyrromethanes (1), and, depending on desired substitution pattern, also employ 1-acyldipyrromethanes (2) and 1,9-diacyldipyrromethanes (3) (Chart 1).1,2 1-Acyldipyrromethanes are readily prepared from the corresponding dipyrromethane, while 1,9-diacyldipyrromethanes can be prepared by 9-acylation of a 1-acyldipyrromethane or by 1,9-diacylation of a dipyrromethane. Although the acylation procedures work reasonably well, purification is difficult owing to the lack of crystallinity of the acyldipyrromethanes. Accordingly, the mixture containing the acyldipyrromethane is usually separated by chromatography, which can be tedious owing to the tendency of the acyldipyrromethanes to streak on chromatographic media.

One of our objectives over the past few years has been to increase the scale of porphyrin syntheses, which entails decreasing if not eliminating reliance on chromatography for purification. Toward this goal, we recently developed a simple procedure for isolating a 1,9-diacyldipyrromethane from the diacylation reaction mixture by forming a dialkyltin complex (Chart 2).7 Dipyrromethanes, 1-acyldipyrromethanes or 1,8-diacyldipyrromethanes did not give tin complexes. The tin complex of a 1,9-diacyldipyrromethane was hydrophobic and crystalline, greatly facilitating isolation. In addition, the tin complex readily underwent decomplexation upon treatment with dilute trifluoroacetic acid. The availability of the tin-complexation procedure has enabled routine synthesis of multigram quantities of 1,9-diacyldipyrromethanes.
